This invention relates generally to interferometrically controlled stages movable along X and Y axes for positioning or aligning a first object, such as a photomask or a semiconductive wafer, with respect to a second object, such as a reticle, or an image thereof, and more specifically to an interferometrically controlled stage movable along precisely orthogonal X and Y axes for successively positioning or aligning different regions of a photomask or a semiconductive wafer with respect to the same reticle or an image thereof.
In the semiconductor industry interferometrically controlled stages movable along X and Y axes are employed both in the fabrication of photomasks and in the processing of semiconductive wafers to form integrated circuits and the like. A high (submicron) resolution photomask is typically fabricated by employing such an inteferometrically controlled stage to successively position different regions of the photomask with respect to a reticle, or an image of a reticle, representing a level of microcircuitry to be printed on the photomask at each of those regions. This step-and-repeat printing operation forms an array of adjacent regions of microcircuitry of one level on the photomask in rows and columns parallel to the X and Y axes of motion of the interferometrically controlled stage. A set of such photomasks, each bearing an array of microcircuitry of a different level is typically employed in the fabrication of integrated circuits or the like from a semiconductive wafer. In the course of this fabrication, the semiconductive wafer is sequentially aligned with each photomask of the set and the level of microcircuitry printed on the photomask is in turn printed on the semiconductive wafer. However, it is also possible to eliminate the operation of forming a set of such photomasks by employing an interferometrically controlled stage to successively align different regions of the semiconductive wafer with each of the reticles employed in fabricating the set of photomasks so that the level of microcircuitry represented by each of those reticles may be printed directly on the semiconductive wafer at each of those regions during separate step-and-repeat printing operations.
In order to facilitate the precise positioning or alignment of an array of adjacent regions of one level of microcircuitry being printed on a photomask, or on a semiconductive wafer, relative to each array of adjacent regions of microcircuitry of another level previously printed or yet to be printed on the other photomasks of the same set, or relative to each array of adjacent regions of microcircuitry of another level previously printed or yet to be printed on the semiconductive wafer, it would be highly desirable to employ an interferometerically controlled stage having precisely orthogonal X and Y axes of motion for step-and-repeat printing operations such as those described above. Unfortunately, however, conventional interferometrically controlled stages do not have precisely orthogonal X and Y axes of motion. Moreover, the degree of nonorthogonality of the X and Y axes of motion of such stages is normally different from stage to stage so that different stages have different frames of reference and cannot therefore be employed interchangeably in printing different levels of microcircuitry on different photomasks of the same set or on the same semiconductive wafer or batch of semiconductive wafers.
Conventional interferometrically controlled stages typically employ a separate interferometer system for each axis of motion of the stage with a first movable mirror of the interferometer system for the X axis of motion being mounted on the stage in a vertical plane normal to the X axis of motion and with a second movable mirror for the Y axis of motion being mounted on the stage in a vertical plane normal to the Y axis of motion, as shown in British Pat. No. 1,196,281 entitled INTERFEROMETRICALLY CONTROLLED POSITIONING APPARATUS and published on June 24, 1970. Since these mirrors must be disposed in vertical planes precisely orthogonal to one another for the stage to have precisely orthogonal X and Y axes of motion, special measurement equipment and procedures involving considerable effort and expense are employed to mount and maintain these mirrors in vertical planes as closely orthogonal to one another as possible. However, since even the best measurement equipment has a finite accuracy, it is in fact not possible to mount and maintain these mirrors in precisely orthogonal vertical planes. As a result the stage does not have precisely orthogonal X and Y axes of motion.
Accordingly, it is the principal object of this invention to provide an interferometrically controlled stage have precisely orthogonal X and Y axes of motion.
Another object of this invention is to provide such a stage while eliminating the effort and expense in attempting to mount the first and second movable mirrors in precisely orthogonal vertical planes.
Still another object of this invention is to provide such a stage which may be employed interchangeably with other such stages in printing different levels of microcircuitry on different photomasks of the same set or on the same semiconductive wafer or batch of semiconductive wafers.
These and other objects are accomplished according to the illustrated preferred embodiment of this invention by employing a stage movable along X and Y axes in a horizontal plane, and by fixedly mounting first and second movable plane mirrors of first and second interferometer systems, respectively, on the stage in vertical planes intersecting one another at the Y axis with the first and second movable mirrors symmetrically disposed about the Y axis. First and second stationary plane mirrors are fixedly mounted above the stage on a housing of a projection lens or some other such utilization device and are disposed parallel to the first and second movable mirrors, respectively. The first interferometer system has a first measurement path normal to the first movable mirror and a first reference path normal to the first stationary mirror. As the stage is moved along either the X or the Y axis, the first interferometer system produces a first measurement signal indicative of the velocity of the first movable mirror while it is being moved (relative to the first stationary mirror) along the first measurement path. Similarly, the second interferometer system has a second measurement path normal to the second movable mirror and a second reference path normal to the second stationary mirror. As the stage is moved along either the X or the Y axis, the second interferometer system produces a second measurement signal indicative of the velocity of the second movable mirror while it is being moved (relative to the second stationary mirror) along the second measurement path. In response to differences and sums of these first and second measurement signals, first and second position control circuits move the stage along precisely orthogonal X and Y axes with the Y axis bisecting the angle between the first and second movable mirrors. Thus, the stage is provided with precisely orthogonal X and Y axes of motion without requiring the first and second movable mirrors to be mounted in precisely orthogonal vertical planes and without requiring any other such unattainable relationship between these mirrors or other parts of the stage. This eliminates the principal source of degradation in the orthogonality of the X and Y axes of motion of the stage. By comparison, other sources of degradation, such as unevenness of the first and second movable mirrors, are insignificant and are therefore disregarded for purposes of this application.